Vibrating measuring devices such as, for example, densitometers and Coriolis flow meters are used for measuring a characteristic of flowing substances, such as, for example, density, mass flow rate, volume flow rate, totalized mass flow, temperature, and other information. Vibrating measuring devices include one or more conduits, which may have a variety of shapes, such as, for example, straight, U-shaped, or irregular configurations.
The one or more conduits have a set of natural vibration modes, including, for example, simple bending, torsional, radial, and coupled modes. At least one drive vibrates or drives the one or more conduits at a resonance frequency in one of these drive modes for purposes of determining a characteristic of the flowing substance. One or more electronics transmit a sinusoidal or a square wave drive signal to the at least one drive, which is typically a magnet/coil combination, with the magnet typically being affixed to the conduit and the coil being affixed to a mounting structure or to another conduit. The drive signal causes the drive to vibrate the one or more conduits at the drive frequency in the drive mode. For example, the drive signal may be a periodic electrical current transmitted to the coil.
At least one pick-off detects the motion of the conduit(s) and generates a sinusoidal pick-off signal representative of the motion of the vibrating conduit(s). The pick-off is typically a magnet/coil combination, with the magnet typically being affixed to one conduit and the coil being affixed to a mounting structure or to another conduit. The pick-off signal is transmitted to the one or more electronics; and according to well known principals the pick-off signal may be used by the one or more electronics to determine a characteristic of the flowing substance or adjust the drive signal, if necessary.
In order to drive a vibrating measuring device in a desired mode, a drive chain is typically employed. The drive chain modifies one or more pick-off signals to generate the drive signal. The drive chain reinforces the appropriate drive frequency and suppresses other drive frequencies. By way of an example, a generated drive signal may start with a pick-off signal. This pick-off signal may then be modified, for example, filtering out undesired modes, adjusting for signal gain, and phase shifting, to provide the generated drive signal.
Depending on operating conditions, a given vibrating measuring device may operate more accurately at certain frequencies. For example, certain vibrating measuring devices are capable of operating at either a low frequency first bend drive mode or a high frequency second bend drive mode. The low frequency first bend drive mode may provide better entrained air performance and the high frequency second bend drive mode may provide more accurate measurements across a wider range of operating conditions.
In situations where vibrating measuring devices are designed to operate in multiple modes, as a practical matter, it has been difficult to switch between modes because a single fixed drive chain is not capable of generating more than one mode. This is particularly problematic where any analog hardware, due to its fixed nature, is included as a component of the drive chain.
The present invention is directed to overcoming this disadvantage inherent in prior single conduit systems.